For the formation of unobjectionable starting stitches, it is required to clamp the needle threads or provide them of corresponding length at the beginning of sewing. When the starting thread is too long, it projects from the workpiece and can be engaged by the stitch-forming tools during the subsequent stitch-forming cycles so that an unclean seam results and a subsequent cutting of the starting thread is required. When the starting thread is too short, dropped stitches can result from the raveling of the thread. The correct starting thread length is difficult to determine especially when endless multifilament and most especially when monofilament sewing threads are used since these sewing threads have special elongation characteristics. To prevent a raveling of the needle thread with very short needle threads, controlled thread clamping devices are known which clamp the needle thread for the formation of objection-free starting stitches.
The German utility model 6 603 307 provides, for this purposes, a controlled thread-clamping device for the needle thread of a stitch group or cluster sewing machine which is brought automatically to a standstill at the end of a working cycle, whereby the fabric clamp foot is released, the thread separated and the needle bar briefly held at the upper deadpoint. With such a controlled thread clamping device, it is a disadvantage that the needle thread, although clamped, can because the clamp is arranged directly on the needle bar, only be prevented from unthreading. The length of the starting thread is here not determinable.